power: supply: axp20x_usb_power: enable USB BC detection on AXP813

The AXP813 PMIC has support for detection of USB Battery Charging
specification, and it will limit the current to 500mA by default when
the detection is not enabled or the detection result is SDP.

Enable the BC detection to allow correctly selection of the current.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>